The present invention is concerned with improving the gain stability of microchannel plate (MCP) electron multipliers.
Microchannel plates are increasingly being used in image intensifiers, radiation detectors, CRT display systems, and other applications because of the unique combination of properties they possess. These include high operating gain, low noise, high spatial resolution, and large active areas coupled with compact size.
A microchannel plate (MCP), also known as a channel-electron multiplier array (CEMA), consists of a parallel array of individual electron multiplier channels of microscopic diameter. MCP's are usually made of glass as a polygonal or round disk about 20 to 50 mm in diameter and about 0.6 to 4 mm thick. Channel diameters typically are in the range of about 12 to 100 microns. Various methods are used to manufacture microchannel plates, the most widely used of which are based on glass fiber drawing techniques similar to those used to make fiber optic plates. A detailed description of channel plate manufacturing technology may be found in Acta Electronica, Vol. 14, No. 2 (1971) at pages 201-224. Briefly, however, a suitable matrix glass is first drawn into tubular fibers, which either may be hollow or may contain a metal or soluble glass core. Lengths of the fiber are formed into a parallel bundle, then fused together by applying pressure to the bundle and heating it to a temperature of about 500.degree.-600.degree. C. Channel plates are made by cutting the fused bundle into slices and polishing the faces of each slice. If the bundles are formed using cored fibers, the cores are dissolved out with an etchant at this point in the procedure. The hollow channels are next treated to obtain the necessary electrical conductance and secondary emission properties required for channel electron multiplication. Finally, metal electrodes are applied to both faces of the plate by vacuum deposition.
MCP's with channel diameters smaller than about 40 microns are produced by a double draw method similar to the process just described, except that thicker fibers are used initially. Long, narrow bundles are assembled and fused together, typically in a hexagonal array. The fused bundles are then drawn a second time to produce multifiber units in which each channel is of the required final size. Finally, after the hexagonal multifiber is cut into lengths, packed into bundles and fused together, channel plates are made from the fused bundle in the manner already described.
A microchannel plate is operated in a vacuum with different potentials applied to the electrodes to produce an axial electric field through the channels. When radiation in the form of electrons, photons, x-rays, etc. enters the low potential end of a channel and strikes the inner surface with sufficient energy, electrons are emitted from the surface. (The channel typically are tilted or curved a few degrees from normal to prevent radiation from passing straight through.) The emitted electrons collide with the walls repeatedly as they are accelerated toward the output end of the channel by the applied electric field, producing additional secondaries. Ultimately, very large numbers of electrons produced by such multiplication are emitted from the high potential end of the channel.
The gain of a channel multiplier depends on its length-to-diameter ratio, on the magnitude of the applied potentials, and on the secondary emission characteristics of the semiconducting inner wall surface. While larger diameter, single channel electron multipliers of the Channeltron type have a long period of stable gain in operation, this characteristic has not been shared by microchannel plates. Gain degradations of one to two orders of magnitude have been reported, for example, by Sandel et al., Applied Optics, Vol. 16, No. 5 (May, 1977) and Authinarayanan et al., Advances in Electronics and Electron Physics, Vol. 40A pp. 167-181. Academic Press (1976).
The glass commonly used to make MCP's (e.g. Corning 8161) is basically a potash lead glass, which is a good insulator. The necessary electrical conductance and secondary emission properties are developed by heating the channel plates in hydrogen to produce a very thin semiconducting surface film on the channel walls. The mechanism of secondary emission from the glass channel walls is not well understood. It has been shown, however, that potassium is present on the secondary emission surfaces in disproportionately large quantities, and that its concentration affects secondary electron yield. For example, see Siddiqui, J. Appl. Optics Volt. 48, No. 7 (July, 1977) and Hill, Advances in Electronics and Electron Physics, Vol. 40A, pp. 153-165, Academic Press (1976). A decrease in channel surface potassium concentration has been found to result from prolonged electron bombardment, and suggested as a possible cause of MCP gain degradation.